Waveguide structure for microwave linear electron accelerator



July 18, 1961 M. G. KELLIHER ETAL 2,993,143

WAVEGUIDE STRUCTURE FOR MICROWAVE LINEAR ELECTRON ACCELERATOR 4 Sheets-Sheet 1 Filed Dec. 30, 1955 July 18, 1961 M. G. KELLIHER ETAL 2,993,143

WAVEGUIDE STRUCTURE FOR MICROWAVE LINEAR ELECTRON ACCELERATOR 4 Sheets-Sheet 2 Filed Dec. 50, 1955 July 18, 1961 M. G. KELLIHER ETAL 2,993,143

WAVEGUIDE STRUCTURE FOR MICROWAVE LINEAR ELECTRON ACCELERATOR Filed Dec. 50, 1955 4 Sheets-Sheet 3 n u n n n n n n n n Mil ll ll ll ll ILII "1| July 18, 1961 M. G. KELLIHER ET AL 2,993,143

WAVEGUIDE STRUCTURE FOR MICROWAVE LINEAR ELECTRON ACCELERATOR Filed Dec. 50, 1955 4 Sheets-Shet 4 Patented July 18, 1961 2 993 143 WAVEGUIDE srnucrunn non MICROWAVE LIN- EAR ELECTRGN ACCELERATOR Maurice G. Kelliher, Ariington, John C. Nygartl, Lexington, Bernard G. E. Stifi, Lynnfield Center, and Lester E. Wilson, Jr., Bedford, Mass, assignors to High Voltage Engineering Corporation, Cambridge, Mass, a corporation of Massachusetts Filed Dec. 30, 1955, Ser. No. 556,672 5 Claims. (Cl. 315--39) This invention relates to microwave linear accelerators, and in particular to a novel accelerating waveguide structure for a microwave linear accelerator, together with a novel method of manufacturing such an accelerating waveguide structure.

In a microwave linear accelerator electrons are injected into one end of a waveguide in which a traveling electromagnetic Wave is produced by means of high-frequency power fed into the Waveguide from a high-frequency source such as a magnetron or klystron. The apparatus is designed so that the electromagnetic wave has an axial electric field component, and the waveguide is loaded so that the phase velocity of the traveling wave is very nearly equal to the velocity of the particles in vacuo. If electrons are injected into the waveguide at a velocity which is almost equal to the velocity of light, the phase velocity of the wave will be 3 1O cm./sec. Some of the electrons will find themselves in an accelerating electric field which raises their velocity to a value very nearly equal to the velocity of light. These electrons can then gain only a negligible amount of additional velocity, and so they remain in the accelerating field throughout the length of the Waveguide, continually gaining energy from the wave in the form of an increase in the electrons mass. In order to compensate for defocussing forces and to maintain the accelerated electrons in the form of an electron beam, focusing magnets are provided along the length of the accelerating waveguide.

The required loading of the accelerating waveguide may be provided in various ways. For example, the accelerating waveguide may be constructed of suitable dielectric material. Alternatively, an accelerating waveguide constructed of a suitable conductive material may be irisloaded. These and other means for loading the accelerating waveguide are well-known in the microwave art.

The invention comprehends an accelerating waveguide structure in which an accelerating waveguide is supported by a tubular member which surrounds theaccelerating waveguide. If focusing magnets are employed, said tubular member may comprise the core upon which the focusing magnets are supported, said core being hollow and surrounding the accelerating waveguide. The magnet core thus provides the mechanical support for the accelerating waveguide as Well as for the focusing magnets. In accordance with the invention, the magnet core or other tubular support has an inner diameter greater than the outer diameter of the accelerating Waveguide, so as to form an annular space between the accelerating waveguide and the magnet core, through which a suitable liquid is circulated in order to dissipate not only the heat generated in the accelerating waveguide by the currents therein and electron bombardment thereof, but also the heat generated in the focusing magnets by the currents passing therethrough.

In a modified embodiment of the invention, the magnet core holds not only the accelerating waveguide but also at least one of the radio-frequency couplings, so that the radio-frequency coupling is an integral part of the a ccelerating waveguide structure, as will be described in detail hereinafter. V

The invention also comprehends a novel method of manufacturing such an accelerating waveguide structure, in which an accelerating waveguide, comprising alternating rings and aperture disks, is compressed while a magnet core or other tubular support is attached thereto, so that, upon release of the means for compressing the accelerating waveguide, the magnet core provides the necessary support for the accelerating waveguide. Alternatively, and without departing from the spirit and scope of the invention, the magnet core may be expanded While the accelerating waveguide is fitted within the core, so that, upon release of the means for expanding the magnet core, the core shrinks so as to compress the accelerating waveguide. Preferably both the foregoing techniques are employed, and during assembly the waveguide is compressed while the core is expanded. In the resultant accelerating waveguide structure, the alternating rings and apertured disks of the accelerating waveguide are held in compression by the magnet core or other tubular support, the magnet core being held in tension. The cooling liquid which is circulated, in accordance with the invention, in the annular space between the accelerating waveguide and the magnet core prevents thermal expansion of the magnetic core when the accelerating Waveguide structure is used to accelerate charged particles.

The invention may best be understood from the following detailed description thereof, having reference to the accompanying drawings in which:

FIG. 1 is a longitudinal central section of an accelerating waveguide structure constructed in accordance with the invention;

FIG. 1A is a detail of a portion of the apparatus of FIG. 1, but showing a modification of the radio-frequency coupling of the apparatus of FIG. 1;

FIG. 2 is a transverse section taken along the line 22 of FIG. 1;

FIG. 3 is an end elevation of the apparatus of FIG. 1;

FIG. 4 is a somewhat diagrammatic view, partly in side elevation and partly in vertical section, of apparatus for assembling the accelerating Waveguide structure of FIGS. 13 in accordance with the invention;

FIG. 5 is a view similar to that of FIG. 4, and showing a step in the assembly subsequent to that shown in FIG. 4; and

FIG. 6 is a view similar to that of FIG. 1, but showing the accelerating waveguide structure prior to completion of assembly.

Referring to the drawings, and first to FIGS. 1, 2 and 3 thereof, the accelerating waveguide 31 of a microwave linear accelerator constructed in accordance with the invention may compromise a multiplicity of alternating copper spacer rings 2 and copper apertured disks 3, all having the same external diameter. Radio-frequency power is fed into the accelerating waveguide 1 through a radio-frequency coupling 4, and creates a traveling electromagnetic wave in the accelerating waveguide 1 of such a nature that electrons, injected into the accelerating waveguide 1 in an appropriate manner, are accelerated along the longitudinal axis of the accelerating waveguide 1. In order to keep the accelerated electrons in the form of a beam, focusing coils 5 are mounted on a core 6 which in turn is mounted on the accelerating waveguide 1 in the manner shown in FIGS. 1 and 2. V

In order for the apparatus'shown in FIGS. 1, 2 and 3 to function properly as a microwave linear accelerator, various conditions must be satisfied. Since the path of the electrons along the accelerating waveguide 1 must be unobstructed, the interior of the accelerating waveguide 1 must be evacuated, and therefore vacuum-tight seals must be provided between the rings 2 and. the disks I therefore good radio-frequency seals must be provided 3 between the rings 2 and the disks 3. Thirdly, the currents in the Walls of the accelerating waveguide 1 create heat which must be dissipated. Fourthly, the currents in the focusing coils 5 create heat which must be dissipated. And lastly, the accelerating waveguide 1 must form a rigid structure which is mechanically strong.

By means of the invention, all these requirements are satisfied in a novel way. In accordance with the invention, the core 6 constitutes the principal mechanical support of the apparatus shown in FIGS. 1, 2 and 3. The accelerating waveguide 1 is supported within the core 6, as shown in FIGS. 1 and 2, the accelerating waveguide 1 being under compression and the core 6 being under tension. The longitudinal compression of the accelerating Waveguide 1 assists in providing the necessary rigidity, vacuum tightness, and radio-frequency seal.

In accordance with the invention, the inner diameter of the core 6 is somewhat larger than the outer diameter of the accelerating waveguide 1, so that an annular space 7 is provided between the core 6 and the accelerating waveguide 1. This space 7 is used to cool the accelerating waveguide 1 and the focusing coils 5 simultaneously, by causing a suitable liquid, such as water, to flow therethrough. In order to utilize as much of the space 7 as possible for cooling purposes, two helical wires 8 may be mounted within the space 7, as shown in FIG. 2, so as to create two helical channels in the space 7 through which the water or other liquid flows from an input pipe '9 to an output pipe 10. The wires 8 extend all the way to that end of the annular space 7 at which the input pipe 9 and output pipe 10 are located, so that water is fed from the input pipe 9 into one helical chan-.

nel and into the output pipe 10 from the other helical channel. The wires 8 do not extend all the way to the opposite end of the annular space 7, thereby providing a channel through which the water or other liquid can flow from one helical channel into the other. The wires 8 may be of stainless steel one-quarter inch in diameter, and the helices may make one or more turns along their length. The wires 8 may be tack-welded or silverbrazed to the core 6. As much as one-sixteenth inch clearance may be provided between the wires 8 and the walls of the annular space 7, since the wires 8 serve merely to guide the water through the space 7. An 0- ring 11 may be provided, as shown in FIGS. 1 and 6, to ensure a water-tight seal. Other arrangements may be made for conducting the cooling liquid through the space 7 without departing from the scope of the invention.

The method of manufacturing the apparatus of FIGS. 1, 2 and 3 in accordance with the invention may best be understood with reference to FIGS. 4, 5 and 6 in addition to FIGS. 1, 2 and 3. Referring now to FIG. 4, the multiplicity of rings 2 and disks 3 which comprise the accelerating waveguide 1 are stacked within suitable guide bars 12. To improve the vacuum-tight seal, the outer surface of the accelerating waveguide 1 may be coated with a suitable material, such as an epoxy resin, at the junctions between the rings 2 and the disks 3.

After the accelerating waveguide 1 has been assembled in the foregoing manner, a ram 13 adapted to fit the upper end of the accelerating waveguide 1 is lowered in place and compresses the accelerating waveguide 1,,

as shown in FIGS. 4 and 5. Then, the guide bars 12 are removed and the core 6, upon which the focussing coils 5 have been mounted, is lowered over the accelerating waveguide 1, as shown in FIG. 5. The length of the core 6 is such that, when the upper flange 14 of the core 6 restson the upper end of the accelerating waveguide 1, the bottom flange 15 is above the bottom end.

of the accelerating waveguide '1, as shown in FIGS. 5

and 6.

It has already been stated that the ram 113 compresses the accelerating Waveguide 1. However, in the preferred method of manufacturing the accelerating waveguide in accordance with the invention, the bottom flange 15 nevertheless does not quite extend below the bottom end of the accelerating waveguide 1, despite such compression by the ram 13. In accordance with the invention, current is caused to flow through the focussing coils 5, thereby heating the core 6 and causing it to expand longitudinally. Owing to the fact that there is little actual contact between the core 6 and the accelerating waveguide 1, the heat generated in the core 6 is not readily transmitted to the accelerating waveguide 1 during assembly, and a temperature difference between them of F. may easily be obtained. The resultant longitudinal expansion of the core 6 relative to the accelerating waveguide 1 is sufficient to bring the bottom flange 15 of the core 6 flush with the bottom end of the accelerating waveguide 1, as shown in FIG. 1. While the core 6 is so expanded, an outer flange 16 is bolted to the bottom flange 15 of the core 6, as shown in FIG. 1.

In accordance with the invention, the radio-frequency coupling 4 is sandwiched between the bottom flange 15 of the core 6 and the outer flange 16, as shown in FIGS. 1, 3 and 6. In this way, the radio-frequency coupling 4 is made an integral part of the accelerating waveguide structure. The cone 17 of the radio-frequency coupling 4 may comprise ferromagnetic material which has been copper-plated on its inner surface as shown at 13 in FIG. 1A. The use of ferromagnetic material in the cone 17 of the radio-frequency coupling 4 not only increases the magnetic flux produced in the accelerating waveguide 1 by the focusing coils 5 at that end of the acceleration waveguide 1, but also shields the electron gun which injects electrons into the accelerating waveguide 1 from the magnetic field of the focusing coils 5.

After the outer flange 16 has been bolted as described, the ram 13 is removed and the current through the to cusing coils 5 is turned off. The accelerating waveguide 1 then tends to expand, while the core 6 tends to contract, thereby effectively creating a rigid, mechanically strong structure.

During actual operation of the microwave linear accelerator, the cooling liquid which flows through the annular space 7 prevents the core 6 from expanding owing to the heat created by the current through the focusing coils 5.

While the preferred method of manufacturing an accelerating Waveguide in accordance With the invention comprises compressing the accelerating waveguide and expanding the core during assembly, the invention also includes methods of manufacturing accelerating waveguides in which only one of these steps is employed. Moreover, the invention is not limited to the use of mechanical compression of the accelerating waveguide and thermal expansion.

of the core, but includes other well-known techniques for such compression and expansion, such as cooling the accelcrating waveguide, mechanical stretching of the core, etc.

Having thus described the accelerating waveguide structure of the invention, together with the method of manufacturing the same in accordance with the invention, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense, and not for purposes of limitation, the scope of the invention being set forth in the following claims.

We claim:

1. An accelerating waveguide structure for a microwave linear electron accelerator comprising in combination: an iris-loaded waveguide comprising a multiplicity of alternating spacer rings and apertured disks; a tubular member laterally enclosing said waveguide in such a manner that an annular space is provided between the inner wall of said tubular member and the outer wall of said waveguide, said tubular member having flanges attached thereto which abut against the ends of said waveguide in such a manner as to support said waveguide; and means for circulating a fluid through said annular space to cool said waveguide.

2. An accelerating waveguide structure for a microwave linear electron accelerator comprising in combination: an iris-loaded waveguide comprising a multiplicity of alternating spacer rings and apertured disks; a hollow magnet core upon which are supported the focussing coils of the microwave linear accelerator, said magnet core laterally enclosing said waveguide in such a manner that an annular space is provided between the inner wall of said magnet core and the outer wall of said waveguide, said magnet core having flanges attached tthereto which abut against the ends of said waveguide in such a manner as to support said waveguide; and means for circulating a fluid through said annular space to cool said waveguide and said magnet core.

3. An accelerating waveguide structure for a micro wave linear electron accelerator comprising in combination: an iris-loaded waveguide comprising an assembly of a multiplicity of alternating spacer rings and apertured disks together with a radio-frequency coupling at one end of said assembly and a hollow magnet core upon which are supported the focussing coils of the microwave linear accelerator, said magnet core laterally enclosing said waveguide and having flanges attached thereto which abut against the ends of said waveguide in such a manner as to hold said waveguide in compression while said magnet core is held in tension, said radio-frequency coupling including a cone, comprising ferro-magnetic material, whose exterior surface forms part of the boundary of said radio-frequency coupling and whose interior surface forms a tubular passageway adapted to transmit the electron beam of said microwave linear accelerator during operation thereof, and means for feeding radio-frequency power into said waveguide through said radio-frequency coupling.

4. A11 accelerating waveguide structure for a microwave linear electron accelerator, comprising in combination: an iris-loaded waveguide comprising a multiplicity of alternating spacer rings and apertured disks; a hollow magnet core upon which are supported the focussing coils of the microwave linear accelerator, said magnet core laterally enclosing said waveguide in such a manner that an annular space is provided between the inner wall of said magnet core and the outer wall of said waveguide; said magnet core having flanges attached thereto which abut against the ends of said waveguide in such a manner as to hold said waveguide in compression while said magnet core is held in tension; and means for circulating 6 a liquid through said annular space to cool said waveguide and said magnet core.

5. An accelerating waveguide structure for a microwave linear accelerator, comprising in combination: an iris-loaded waveguide comprising a multiplicity of alternating spacer rings and apertured disks; a hollow magnet core upon which are supported the focussing coils of the microwave linear accelerator, said magnet core laterally enclosing said waveguide in such a manner that an annular space is provided between the inner wall of said magnet core and the outer wall of said waveguide; and means for circulating a fluid through said annular space to cool said waveguide and said magnet core, wherein said means for circulating a liquid through said annular space comprises at least two helical wires supported within said annular space so as substantially to prevent the flow of said liquid in said annular space transverse to said wires and extending from a first end of said annular space almost to the second end thereof, whereby said annular space is divided into at least two helical channels which are in communication at the second end of said annular space; and wherein liquid is injected into one of said channels at the first end of said annular space and removed from the other of said channels at the second end of said annular space.

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